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Old 9th January 2005, 03:04 PM   #1
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Default Tubes and magnetic fields

I've been sort of puzzled by anecdotal reports of the effects of demagnetization on tubes. The hypothesis is that residual magnetism might cause the electron paths to be deflected, changing gain, transconductance, and distortion.

Now, to be clear, there's no actual evidence of the audibility of demagnetization, just anecdotes. But hey, that's where we start. From a physics point of view, this is puzzling since the effects of magnetic fields are small (relative to electric fields) and the electron stream in most tubes has an extraodinarily high velocity- relativistic, in fact. Unlike a CRT, the electron path is VERY short inside a tube, so there's not much time to deflect- the curvature has to be quite strong to be significant. Physics models aside, I'm curious as to whether any measurable change could occur via demagnetization.

As usual, I was limited to what I have on hand and that does not include a demagnetizer. So I went about this the opposite way- I set up a tube in a jig, measured gain and distortion in a "raw" tube, then repeated the measurements in the presence of a magnetic field.

Since I'm elbow deep in ECC88 measurements, I used that as a guinea pig. The test setup was what I showed in the "Getting Jiggy" thread: a cascode-CCS-loaded voltage amp with LED biasing. The current was set to 8.0mA, cathode bias at 1.7V, 40V p-p output, 140V B+. The tube was a Siemens ECC88, date code 887. Measurement repeatability throughout these tests has been about +/-0.5 dB.

Initial measurements:

Gain = 21.5 (all the Siemens ECC88s I tested had this low out-of-spec gain)
2nd HD = -41.7dB
3rd HD = -73.1dB
4th HD = -73.7dB

The initial measurements were repeated before and after each of the following measurements to check for drift and error. All control measurements fell within +/-0.5dB of the initial measurement.

First magnetic field measurement:

A refrigerator magnet (from a Teletubbies set, Tink Winky, purple) was placed on the side of the tube and gain/distortion were remeasured.

Gain: 21.5
2nd HD = -41.6dB
3rd HD = -72.8dB
4th HD = -74.1dB

The magnet was placed on the top of the tube to see if there is any geometric effect:

Gain: 21.5
2nd HD = -41.8dB
3rd HD = -73.0dB
4th HD = -74.2dB

Hmm, nothing significant. Could it be a function of strength? I repeated the measurement, but substituted a magnet taken from a blown Dynaudio D-28; this magnet has a FIERCE pull and readily attracts every nut and bolt within several centimeters. As before, the magnet was placed directly touching thre tube envelope. Results:

Gain: 21.5
2nd HD = -41.8dB
3rd HD = -72.6 dB
4th HD = -74.8dB

As before, no significant difference.

My test was clearly limited to one type of tube, so I make no claim of universality. BUT... lacking either repeatable data showing an effect or controlled listening tests indicating audibility, I would be very skeptical of claims of sonic improvement through demagnetization.
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Old 9th January 2005, 03:12 PM   #2
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Now we need someone with access to Liquid Nitrogen to put the test to the "Cryo tube" craze. -another gimmick that I'm highly skeptical of.
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Old 10th January 2005, 04:42 AM   #3
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Beam tetrodes might not be so forgiving as the ECC88 though. I wonder if the magnetic field messes up the nice shadow the screen grid sits in...
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Old 10th January 2005, 06:10 AM   #4
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Are the grid wires magnetic? What are they typically made of?
Pulling the grids around could have "interesting" effects.
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Old 10th January 2005, 06:33 AM   #5
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No, I don't think the grid wires are magnetic... Would be pretty nasty if they were attracted by the magnet to touch the cathode

I was thinking the magnetic field might deflect electrons around the control grid wires to collide with the screen grid. Normally the screen grid sits in the 'shadow' of the control grid, so screen current is low. The magnetic field might change that, increasing screen current, causing distortion, or something else interesting. Of course I have no idea if this can happen in real life...
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Old 10th January 2005, 07:01 AM   #6
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I have no experience with demagnetising tubes and really no opinion on the effect but would be very surprised if thd measurements showed anything.

If there is a single parameter not worth measuring for audio purposes my choice will be thd. It probably has some audibility once exceeding the 1% range but musically it's really benign.

It is however great for 'myth debunking' as this hopeful post proves:

Quote:
Now we need someone with access to Liquid Nitrogen to put the test to the "Cryo tube" craze. -another gimmick that I'm highly skeptical of.
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Old 10th January 2005, 08:31 AM   #7
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I don't think the magnetic field messes up the small signal triodes like ECC88. The differences in measuring results are caused by the devation in the circuit/tubes/scope by themselves.
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Old 10th January 2005, 08:36 AM   #8
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Quote:
Now, to be clear, there's no actual evidence of the audibility of demagnetization, just anecdotes. But hey, that's where we start.
John Swenson used a spectrum analyser to determine the measurable difference before and after degaussing. His test was on Beam Deflection Tubes so we probably can't extrapolate it to "normal" or other tubes. But I found it an interesting experiment nevertheless.

http://db.audioasylum.com/cgi/m.mpl?...on&r=&session=
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Old 10th January 2005, 09:01 AM   #9
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To SY : Maybe have author of this original claiming " demagnetized brain " - not you, ofcourse .
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Old 10th January 2005, 10:06 AM   #10
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The 6AR8 is most certainly not the 'average' valve....

Have a look at the cross section diagram attached (From the GEC Datasheet)

It looks like the measurement is the spectrum resulting from a ~500Hz input (the 0dB reference). So it seems that it resulted in increased second and third harmonic, while decreasing higher orders and noise.
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